A tergomyan mollusc from the Upper Cambrian of Wales

8 Pages·1994·4.1 MB·English

by  V Berg-Madsen

A TERGOMYAN MOLLUSC LROM THE UPPER
CAMBRIAN OF WALES
by V. BERG-MADSEN and J. S. PEEL
Abstract. Bellerophoncambriensisfrom theUpperCambrianofNorthWalesisredescribedasthetypespecies
ofthe new genus Telamocornu. Unlike most similarly coiled molluscs ofthis age, apertural sinuses are present
which permit both a functional morphological interpretation ofit as an untorted molluscwith the apex ofthe
shall oriented anteriorly, and directly support assignation of Telamocornu to the Order Cyrtonellida of the
Class Tergomya.
The Cambrian was a time of innovation within the Mollusca. From examination of the fossil
record, Yochelson 1963) proposed that a radiation ofmolluscs in the Early Cambrian was eclipsed
(
by a second great diversification near the Cambrian Ordovician boundary, during which most of
the major mollusc groups familiar to us at the present day arose. From the point of view of
anatomy, however, others have suggested that two of these late-appearing, shell-bearing groups,
namely the Monoplacophora and the Polyplacophora, wereancestral stocks appearingmuch earlier
in mollusc evolution (Salvini-Plawen 1985; Wingstrand 1985). Stimulated by models of mollusc
evolution developed originally by Runnegar and Pojeta (1974), the last two decades have seen
attempts to extend the range of the major extant mollusc classes back to the earliest Cambrian
(Runnegar and Pojeta 1985; Yu 1990). These efforts have met with some success but a complex of
mainly Early to Middle Cambrian molluscs currently lies rather uncomfortably within accepted
classifications. This complex includes forms such as the Helcionelloida, elevated to a class by Peel
(1991c/, 19916), the Pelagiellida, the Onychochilida and the Stenothecoida. Most of these groups
were placed within an extended Class Monoplacophora alongside the well-known Neopilina
Lemche, 1957, and its relatives (Runnegar and Pojeta 1974, 1985; Runnegar and Jell 1976),
although this expansion of the concept Monoplacophora has generated discussion concerning the
usefulness of the term in systematics (Wingstrand 1985; see summary in Peel 19916). Thus, Peel
(1991c/, 19916) followed Wingstrand (1985), Haszprunar (1988) and Salvini-Plawen (1985, 1990)
and abandoned the Class Monoplacophora. He introduced a Class Tergomya for members of the
Neopilina group (based on the Subclass Tergomya Horny, 1965). Tergomyans are oriented with the
shall apex in an anterior position, while in the second major group of Lower Palaeozoic untorted
and bilaterally symmetrical molluscs, the extinct Class Helcionelloida, the apex is interpreted as
posterior (Peel 1991c?, 19916).
Classification ofmolluscs within the Cambrian is clearly in a state offlux and this instability can
be expected to persist for some time as current research continues to produce startling discoveries.
Not least the recent description ofarticulated halkieriids from the Lower Cambrian ofGreenland
(Conway Morris and Peel 1990) and the re-interpretation ofCambrian ‘scaly-shelled’ molluscs by
Bengtson 1992) open a new perspective to early molluscan evolution. The abundant microsclerites
(
and the associated larger anterior and posterior plates in the halkieriids are shell features which find
morphologic equivalents in the aplacophoran and polyplacophoran molluscs which Salvini-Plawen
(1985) placed in the ancestry of the later shelled forms. Moreover, halkieriids occur in strata of
suitable age; aplacophorans have no fossil record and that of polyplacophorans in the Cambrian
is scant.
The group of isostrophically coiled forms referred to as the bellerophontiform molluscs has a
|Palaeontology, Vol. 37, Part 3, 1994, pp. 505-512, I pi.] © The Palaeontological Association
PALAEONTOLOGY, VOLUME
506 37
geological range from Cambrian to Triassic (Knight et al. 1960; Yochelson and Yin 1985) and has
traditionally occupied acentral position in discussions ofthe origin ofthe gastropods(Knight 1952;
Yochelson 1967; McLean 1984; Haszprunar 1988; Horny 1991c/). Opinions are divided as to
whether members of this group were gastropods (Harper and Rollins 1982) or untorted molluscs
(Runnegar and Jell 1976; Runnegar and Pojeta 1985). Peel (19916) and others (Yochelson 1967;
Horny 1991c/) considered the bellerophontiform molluscs to be a mixture of both torted and
untorted forms. Forms interpreted as untorted were referred by Peel to the Order Cyrtonellida of
the Class Tergomya, although some genera within the Class Helcionelloida also share this coiling
style (e.g. Protowenella Runnegar and Jell, 1976, and Coreospira Saito, 1936). Supposedly torted
bellerophontiform molluscs were assigned to the Class Gastropoda. Unfortunately, few Cambrian
bellerophontiform molluscs are readily referred to one class or the other on the basis of
morphological features. Features such as an apertural slit producing a median dorsal selenizone,
although previously considered to be a reliable systematic tool, are potentially developed within
unrelated stocks (Rollins and Batten 1968; Yochelson 1984; Haszprunar 1988; Peel 19916).
In this paper we propose a new genus ofbellerophontiform mollusc from localities in the Upper
Cambrian of Wales (Text-fig. 1). Unlike most contemporaneous bellerophontiform species,
apertural structures are present which permit a functional morphological interpretation of this
genus as an untorted mollusc, thus strengthening the record of the Class Tergomya near the
Cambrian-Ordovician boundary.
SYSTEMATIC PALAEONTOLOGY
Class tergomya Horny, 1965
(nom. transl. Peel 1991c/, ex Subclass Tergomya Horny, 1965)
Order cyrtonellida Horny, 1963
Genus telamocornu gen. nov.
Derivationofname. From telamon(Greek), beltorstrap,combinedwith cornu(Latin), horn. ForThomas Belt,
author of the type species.
Type species. Bellerophon cambriensis Belt, 1868.
Diagnosis. Loosely coiled, in early stages open coiled, rapidly expanding but laterally compressed
cyrtonellid tergomyan with about L5 whorls. Dorsum uniformly convex; aperture with broad and
shallow median sinus and tendency to produce umbilical sinuses. Ornamentation of transverse
rugae and fine growth lines, crossed by weak spiral lines.
Discussion. Most ofour knowledge ofLate Cambrian bellerophontiform molluscs stems from two
papers (Knight 1947, 1948) mainly based on North American material. In these papers, six new
genera were described Anconochilus Chalarostrepsis Cloudia Cycloholcits Sinuella and Strepso-
( , , , ,
discus) and Owenella Ulrich and Scofield, 1897 redescribed. He also redescribed Coreospira Saito,
1936, from the Middle Cambrian of Korea and British Columbia but this genus, and possibly also
Cycloholcits, is an unusual helcionelloid.
Ofthegenera listed above, only Anconochilusfrom themiddle UpperCambrian (Franconian)can
.
be closely compared to Telamocornu ofsimilar age; it is distinguished by being more involute and
less rapidly expanding. Ornamentation is poorly known in Anconochilus barnesi Knight, 1947, the
type and only described species, but Knight (1947) recorded a shallow median sinus in the apertural
margin.
Telamocornu closely resembles Sinuitella Yochelson, 1962, from the Lower Ordovician
(Tremadoc) of Norway. It is distinguished, however, by its much larger size (c. 22 mm compared
mm
with 5-8 for Sinuitella norvegica (Brogger, 1882) and greater rate of whorl expansion,
particularly antero-posteriorly. Sinuitella norvegica is open coiled, with the whorls not in contact,
BERG-MADSEN AND PEEL: CAMBRIAN MOLLUSC 507
text-fig. I. Localities ofCambrian tergomyan molluscsdescribed in thetext (modified fromAllenetal. 1981).
text-fig. 2. Telamocornu cambriense (Belt, 1868). Upper Ffestiniog Flags Formation; x3; BM62087. a,
lectotype; fragmentofdorsalsurfaceshowingtransverserugaeandmedian sinus, b,paralectotype; lateral view
ofspecimen crushed in shale.
PALAEONTOLOGY, VOLUME
508 37
whereas Telamocornu is apparently loosely coiled in larger specimens, with shallow impression of
the earlier whorl.
Pharetrolites Wenz, 1943 is similar in general form to Telamocornu but has a lower rate ofwhorl
expansion. This Silurian genus is also distinguished by its deepermedian dorsal sinus and crenulate
ornamentation.
Latouchella aperta Orlowski, 1968 from the Upper Cambrian of the Holy Cross Mountains of
Poland ismore tightlycoiled than Telamocornucambrienseand seemsto have amoreangulardorsal
profile; Orlowski’s original generic assignment is highly questionable.
Jago and Corbett (1990. fig. 2f-j) figured three bellerophontiform molluscs from a topmost
Cambrian sandstone-siltstone sequence in western Tasmania. Their ‘Bellerophontida gen. et sp.
indet. 1 ' resembles Telamocornu in its rate ofexpansion and size, whereas two other indeterminate
forms are more laterally compressed, with an arched dorsal profile.
Telamocornu cambriense (Belt, 1868)
Plate Text-figure 2
1 ;
1868 Bellerophon Cambriensis Belt, p. 11, pi. 2, figs 19-20.
Typematerial. Lectotype (here designated) is the specimen figured by Belt (1868, pi. 2, fig. 20); paralectotype,
the original ofBelt (1868, pi. 2, fig. 19). Both specimens (Text-fig. 2)carry the registration number BM 62087,
and were collected at Craig-y-Dmas, Dolgellau district. North Wales, in strata containing Parabolinoides
bucephalus, indicative ofthe uppermost part of the late Cambrian Ffestiniog Flags Formation.
Other material. A collection (BGS Zv9778, Zv9781, Zv9783-86, Zv9788, Zv9789) made by Stephen Jusypiw
and presented to the British Geological Survey, comes from a fine-grained sandstone occurring at about the
same horizon as the types on the Bryn-llin-fawr forestry road (National Grid Reference SH 7865 3035); the
fauna (PI. I, figs 4-10) is assigned to the Zone of Parabolina spinulosa. Poorly preserved specimens (BGS
RU9708a-b, collected by A. W. A. Rushton), possibly assigned to Telamocornu also occur in the underlying
Maentwrog Formation (cataractes Subzone ofthe Olenus Zone) at Ffridd Dol-y-Moch on the Llanuwchllyn
road, 7 km ESE of Traswfynydd (SH 7648 3298; Allen et al. 1981, p. 308), and in the Dolgellau Member
(Acerocare Zone) of the Cwmhesgen Formation (BGS RU5281, collected by A. W. A. Rushton and S. P.
Tunnicliff) which overlies the Ffestiniog Flags Formation (Ty-newydd-y-mynydd, F5 km E ofRhobell Fawr;
Allen et al. 1981, p. 317 (SH 7996 2601), cf. Rushton, 1982, Text-fig. 1). In terms of the North American
standard, these occurrences range from the upper Dresbachian (Upper Cambrian) upwards to the
Cambrian-Ordovician boundary.
Description. (Based on two suites of specimens (see above), since the types are imperfectly preserved.) Type
species of Telamocornu gen. nov. with about F5 whorls. Earliest growth stage apparently consisting of the
ratherblunt origin ofthecoil, notdistinguishedin terms ofcoiling orornamentation from subsequent growth
stages; the umbilici were probably perforate. In the latest preserved growth stage the whorls appear to be
loosely in contact and the shell is laterally compressed, with width about halfofthe total length. The dorsum
EXPLANATION OF PLATE
1
Figs 1-3. Telamocornucambriense(Belt, 1868)? Poorlypreservedspecimensinlateral view. 1, BGS RU9708A;
MaentwrogFormation; x3. 2, BGS RU9708B; MaentwrogFormation; x 3. 3, BGS RU5281;Cwmhesgen
Formation; showing fine transverse and spiral ornamentation; x2.
Figs 4-10. Telamocornu cambriense (Belt, 1868). Ffestiniog Flags Formation. 4, BGS Zv9788; lateral view to
show theexternal mould oftheearly growth stages; x F5. 5, same specimen in obliquedorsal view to show
the convex dorsum; x F5. 6-7, BGS Zv9789; latex mould and corresponding external mould showing the
umbilico-lateral sinus interpreted asinhalantin function; x8. 8-10, BGS Zv9785; two specimens preserved
as internal moulds with early growth stages broken away. 8, lowermost oftwo specimens figured in 9 ( x4)
showing external mould ofearliest growth stages and internal mould of apertural region with transverse
rugae; x8; 10, dorsal view of upper specimen showing rounded dorsum; x4.
PLATE
1
BERG-MADSEN and PEEL, Telamocornu
PALAEONTOLOGY, VOLUME
510 37
isconvex, uniformly rounded, withoutmedianangulationora keel. In plan view, thelength:width ratioofthe
apertureisabout 3:2. Theapertureissinuatehavinga broad,shallow,emargination in themid-dorsalmargin;
apertural margins appear to be essentially co-planar but in laterally crushed specimens the lateral areas are
adaperturally convex. A tendency for small sinuses to develop on the umbilico-lateral surface, near the suture
with the previous whorl, is observed in a few specimens.
Ornamentationconsistsoftransversegrowthrugae, apparentlyproducedbyperiodicslightexpansionofthe
dorsal apertural margin, and fine growth lines, crossed by weakly developed, fine spiral lines. Shell thickness
and structure are unknown.
Only a portion ofthe dorsal surface is preserved in the lectotype but this clearly shows the median dorsal
sinus; the apparent median band is a result ofcrushing. The paralectotype is crushed in lateral view and also
shows an apparent keel due to fracture along the periphery; the adaperturally convex curvature ofthe lateral
margins is exaggerated by crushing.
The largest ofthe specimens from Bryn-llin-fawr (Zv9788; PI. 1, figs 4-5) shows the early growth stages as
an external mould of the lateral surface, while the partially embedded aperture is preserved as an internal
mould. Notable among specimens from this locality is one (Zv9789) preserved as an external mould of the
lateral surface, showing a sinus on the umbilico-lateral margin (PI. 1, figs 6-7) which is interpreted below as
having served an inhalant function.
FUNCTIONAL MORPHOLOGY OF TELAMOCORNU
Few of the genera and species of bellerophontiform molluscs currently known from the Upper
Cambrian and Lower Ordovician preserve morphological features of the shell which facilitate
interpretation of their systematic affinities either with the Gastropoda or with the Tergomya (the
presence of some helcionelloid taxa within this plexus also cannot be excluded). An important
feature in such interpretations is the presence and distribution of emarginations in the apertural
margin. Sinuitella, as illustrated by Yochelson (1962, pi. 1, fig. 7), preserves a shallow sinus at the
umbilico-lateral shoulder reminiscent of those noted above in Telamocornu cambriense (PI. 1,
figs 6-7; Text-fig. 3). Horny (19916) described similarstructures in Simiitopsisneglecta Perner, 1903
from the Ordovician ofBohemia. Such sinuses are also described in specimens ofyounger age, such
as Pharetrolites bambachi Peel, 1975 from the Silurian Arisaig Group ofNova Scotia (Peel 1975).
text-fig. 3. Reconstruction of Telamocornu cambri-
ense (Belt, 1868) as a tergomyan mollusc, with the
anterior to the left. Water currents (arrows) enter the
mantle cavity anteriorly, by way of the umbilico-
lateral sinus(cf. PI. 1, figs 6-7)located oneachsideof
the shell, prior to exhalation via the posteriormedian
dorsal sinus.
The umbilico-lateral sinuses can be interpreted as the locus of inhalant currents entering the
mantle cavity, with the median dorsal sinus marking the position ofthe posterior exhalant stream
(Text-fig. 3). If the exhalant stream is posterior, the shell is coiled exogastrically, with the initial
whorls located anteriorly, overhanging the anterior margin of the aperture (Peel 19916, p. 19,
fig. 12). Following this reconstruction, Telamocornu (together with Sinuitella, Simiitopsis and
Pharetrolites) isinterpretedasan untorted, exogastric, bilaterally symmetrical mollusc, acyrtonellid
tergomyan in the sense ofPeel (19916). If Telamocornu is interpreted as torted, and assigned to the
Gastropoda, the shell would be endogastric with the earliercoiled portion located posteriorly. This
interpretation, however, is not supported by the presence ofthe supposed inhalant umbilico-lateral
sinuses.
, . . ,
BERG-MADSEN AND PEEL: CAMBRIAN MOLLUSC 511
Acknowledgements Radvan J. Horny (Prague) is thanked for reviewing the manuscript prior to submission,
and for discussions concerningearly univalve molluscs. Ellis L. Yochelson and an anonymous referee offered
valuable criticism. V.B.-M. received financial support from The Royal Society (European Science Exchange
Programme) which enabled her to visit the United Kingdom in 1991. J.S.P. acknowledges support from the
Geological Survey of Greenland, Copenhagen, Denmark. The loan of specimens from the Natural History
Museum, London (numbers prefixed BM) and the British Geological Survey, Keyworth (numbers prefixed
BGS)waskindlyfacilitatedbyN. J. Morris, R. Cleevely,H. C. Ivimey-CookandA. W. A. Rushton;the latter
kindlyprovided informationabout localitiesand stratigraphy, and suggested thegenericname. Mrs K. Bryant
ofthe National Museum ofWales took the photographs.
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V. BERG-MADSEN
Palaeontological Museum
Uppsala University
Norbyvagen 22
S-752 36 Uppsala, Sweden
j. s. PEEL
Institute ofEarth Sciences
Department of Historical Geology and
Palaeontology
Uppsala University
Typescript received 7 May 1993 Norbyvagen 22
Revised typescript received 23 August 1993 S-752 36 Uppsala, Sweden